Watering Plants: How Much and How Often?

How Much Water?

Questions relating to watering plants are probably the most frequently asked ones here at Leafari. “How often should I water? and "How much water should I provide?"

Turns out, there are no hard and fast answers to those questions. There are, however, plenty of clues to finding solid answers. Time to dig out your Sherlock Holmes hat. (Pipe is optional.)

Water + Oxygen = Healthy Plants

Scientists tell us that plant roots need oxygen. The oxygen is used for chemical processes like converting the sugars the plants produce into energy (remember that photosynthesis thing from high school biology?), so plants can grow. If all the spaces between the little soil particles surrounding those roots are filled with water, there's no room for the oxygen. Envision pebbles in a drinking glass. Add water and the air pockets between the pebbles disappear. It's the same with the oxygen in your soil.

Very few plants are happy with consistently wet feet. Short term wetness like the kind delivered by a rainstorm - delightful. Long term soggy soil that never really drains - deadly.

To tell if your soil qualifies as "well drained" watch what happens after a rain storm. If all the puddles created by the storm are gone an hour or so, you're in good shape. If the water is still sitting there three to four hours later, your soil needs help draining.

Takeaway: make sure your soil drains well. If not, tweak it. Mix in material, like compost, to improve drainage. Or, since water always drains downward, elevate the planting area so it can dry out between waterings. Think raised beds. Or containers.

Disappearing Water: Too Much of a Good Thing

Drainage is great. And like all things in excess, too much drainage isn't good. It allows water to pass straight through before plants can absorb much. Now you see it, now you don't. If your soil is sandy and water drains away immediately, it's helpful to add something to help retain moisture. Look to naturally retentive materials like compost or peat moss.

If you're thinking "Wait, didn't you just say the soil needs to drain well?" you're right. It's about finding the mid ground.

Takeaway: Soil that drains right through gives your plants very little time to grab the water they needs. Slow drainage a bit by mixing in water absorbing amendments like compost or peat moss.

Lots of Sunshine Requires Extra Water to Create Balance

You know how you feel after sitting for an hour in the hot sun. Thirsty, right? Your plants feel the same way. More intense sunshine, as in August (versus April) or 10 hours (versus 4 hours), translates into greater thirst. Sunshine increases the amount of moisture that evaporates from the leaf surfaces. This loss, called transpiration, leaves your plants parched and needing to replace that lost water.

Takeaway: The more intense the sunshine, either seasonally or because of the number of hours of sun, the greater your plant's water needs. Be more generous with water as you move into the heat of midsummer.

Plant Size Influences Water Needs

While we rarely stop to think about it, it stands to reason that a mature 6 foot tall dinner plate dahlia plant will need more water than that same dahlia when it’s just 6” tall and starting to grow. The mature plant is losing water from dozens of large leaves as it bakes in the sunshine. The small plant only has a couple of tiny leaves, with very little surface area from which water can evaporate.
But . . . the little plant, because it has a tiny root network, will need water more often. If the young plant's root ball is the size of a tennis ball, once the rain or sprinkler water has drained from that tennis-ball space, the plant is now any water source. This is a consideration for seedlings, which always have tiny root systems and need frequent, as in daily, watering.

Takeaway: The larger the plant, the more water it requires to stay healthy and happy. The younger/smaller the plant, the more frequently it needs water.

Water for the Windblown

Know how a windy day at the beach leaves your skin feeling tight and parched? Plants sited in breezy locations face that situation daily. All that air movement increases water evaporation from their leaves. The plants end up feeling dried out, too.

Takeaway: Plants located in breezy areas typically need more water.

Water Needs That You Can See

Herbaceous plants, as opposed to woody trees and shrubs, are comprised of 70-95% water. Water pressure inside plant cells, known as turgor, helps plants stay upright and stable. The wilting that you’ve seen midday happens because evaporation has reduced the turgor and the plants no longer has enough cell pressure to support the plant's weight. While most wilted plants perk up quickly with a drink, the situation is stressful. And like stressed people, stressed plants rarely look or perform their best.

Takeaway: Your plants will be happier and healthier if you avoid using a wilted profile as the main indicator that it’s time to water. It's helpful to get in the habit of watering plantes early in the day before you head off to work or start other activities.

Was Home Wet or Dry?

Understanding your new plant's origins can provide clues to its ideal growing conditions. Plants that originated in desert-like environments are happiest with little water. For them, more is not better. It's too much. To simplify, choose plants that match the conditions you can offer. Your plants will thrive without lots of extra effort and you’ll have more time to put your feet up and eat bonbons.

If you garden in a warm dry climate, look for plants that originated in warm, dry Mediterranean regions. They'll be a great match for the conditions you can easily offer. This doesn't mean you can't successfully grow plants for other parts of the world, just that they may require more effort to mimic their preferred growing conditions. In other words, not so easy to grow.

Takeaway: The regions where your plant grew in the wild, or their ancestors did, provide excellent clues as to water preference. Some Plants prefer dry settings. Some like it wet. Being generous with water for plants acclimated to desert conditions isn't helpful.

Containers Influence Moisture Needs

Plant containers are made from a wide variety of materials. Containers made from porous materials lose moisture as it evaporates from the side surfaces. This reduces the amount of water available for your plants. In mid-summer for plants in porous containers may need watering twice a day.

Porous materials include:

Unglazed ceramic and terra cotta pots

Woven and moss lined baskets

Wooden planters

Nonporus materials include:

Glazed ceramic pots

Plastic containers

Metal planters

Takeaway: If you're planting in containers, consider what the pot, urn or hanging basket is made of as you evaluate which planter to use where.

Taking the H2O Clues You've Gathered . . .

Start with the assumption that your plant needs an average amount of water, about 1" per week, unless information about the variety selected provides clues to the contrary. These would be descriptive terms like "desert", "Mediterranean", "boggy", etc.

Your plant doesn't care if moisture comes from the sky, a sprinkler system or a watering can. It's all water. An easy way to determine how much water is available is to place a container – an empty tuna or cat food can or yogurt tub – on the ground by your plant/planting site and monitor the amount of water it collects.

What have your observed about the soil drainage where you intend to plant. Slow? Super-fast? Amend the soil before tucking your plant in. Now step back and look at the intended location. Is it super sunny? Windy? That can still work, it just might require a bit of extra water.

Is the plant small, with a yet-to-be-developed root ball? Think daily watering. Or is it large with lots of leaves that will increase evaporation/transpiration? Think deep, generous watering.

Okay, You're Ready!

Dig a hole, tuck your plant in, replace the soil, pat it down and water it well to settle the soil around the roots. The tiny hair roots, which are usually damaged during transplanting, grow back in just a few days. These fine roots, like the pepper plant ones at left, are super heroes, doing the heavy lifting in the water absorption department. Largely unrecognized for their contributions, they are essential for plant health.

If your plant is leafy, provide some shade for the first week while the plant settles in. Shade helps reduce transpiration, aka water loss from the leaves, while the hair roots are reestablishing. Without direct sunlight your plant needs less water during the period when it's not able to gather much. Shade can come from things as simple as a cardboard box placed next to your plant (add a rock on top to anchor the box so it doesn't blow onto your plant) or a patio chair temporarily parked in the garden.

Planting bulbs? Then providing shade is unnecessary. There's no leafy top growth to need TLC while roots reestablish.

Keep an eye on your plant for the first few days, checking it early and late in the day. Add water if the plant looks wilted. As the hair roots develop the plant will be able to absorb the available water and you'll need to add less. In a week, you'll have observed, tweaked and determined just how much your plant needs to be healthy and happy.

Congratulations, Sherlock! You know what to look for and variables to consider. Now no new plant is a complete mystery to you!